Heat treating apparatus



June 23, 1964 J, B 3,138,372

HEAT TREATING APPARATUS Filed Jan. 5, 1962 'FIG. 2

INVENTOR.

J. HOWARD BECK ATTORNEYS United States Patent 3,138,372 HEAT TREATING APPARATUS Jacob Howard Beck, Newton, Mass., assignor to BTU Engineering Corporation, Waltham, Mass, a corporation of Delaware Filed Jan. 3, 1962, Ser. No. 164,012 9 Claims. (Cl. 26337) This invention relates to furnaces for heat treating materials under precisely controlled conditions and more particularly to an improvement in mufiles for furnaces designed to carry out heat treatment of materials in a continuous or intermittent flow system.

This application is a continuation-in-part of my copending application Serial No. 62,670 filed October 14, 1960, for Heat Treating Apparatus, now U'.S. Patent No. 3,041,056. The invention described and claimed in my copending application provides means for establishing a heat barrier between two successive stages operated at different temperatures, the barrier acting to establish and preserve a sharp temperature gradient between the two stages. The invention of my copending application is especially useful where it is desired that materials be brought to a desired heat treating temperature in a minimum time. This is accomplished by first placing the materials in one zone having a temperature considerably in excess of the heat treating temperature, keeping the materials in that one zone for a period of time just sufiicient to raise them to the heat treating temperature, and then transferring them to another zone in which the temperature is at the desired level for heat treatment. The invention of my copending application makes it pos sible to provide in the same muffle two adjacent temperature zones when one zone is at a relatively high temperature and the other zone is at a relatively low temperature, without any substantial transfer of heat from the one zone to the other so as to establish and preserve a very sharp temperature gradient between the two zones.

Notwithstanding the obvious merit of the invention of the aforesaid copending application, it is to be appreciated that there are many Inulti-stage heat treating processes wherein successive stages not only require different temperatures but also require different gaseous atmospheres. In some of these processes, it is imperative that the gaseous atmospheres of succeeding stages be isolated from the other so as to avoid explosive mixtures or contamination. In other cases, it is essential that the limits of the gaseous atmosphere in a stage be defined sharply so as to control precisely the extent of exposure thereto by the materials which are transported through the furnace. This type of control often is required where a chemical reaction occurs. In still other cases, it is necessary not only to isolate the gaseous atmospheres in successive stages, but also to maintain a sharp temperature gradient between the same stages. In most cases these conditions must be met without the use of barriers, auxiliary devices, or transformations in mufile configuration which will impede uniform distribution of the heat-treating gas in a stage or complicate the means or mode of conveying materials from one stage to the other or unduly extend or restrict the length or size of the muffie. Heat treatment processes requiring the use of gaseous atmospheres are involved in many different industries, but they are especially critical in the manufacture of electronic components such as semiconductors and magnetic and resistive metal films where only a slight deviation from a specified operating condition or technique can result in a large drop in useful output, i.e., yield. Unfortunately, although the invention of my copending application provides an exceptionally effective means for separating two successive heating zones temperature-wise,

it is not an effective barrier for preventing the gaseous atmosphere in one heating zone from migrating into an adjacent heating zone.

Accordingly, the primary object of the present invention is to provide an improvement in mufile construction which facilitates execution of multi-stage heat treatment processes involving heating in gaseous atmospheres, particularly processes of the type where yield is dependent upon the level of control over individual operating conditions. As used herein, the term multi-stage includes two or more stages distinguishable one from the other by some specific difference in environment, such as a difference in temperature and/ or atmosphere.

A more specific object of the invention is to provide a muflle for a heat treatment furnace which comprises first and second muffie sections adapted tobe heated to the same or different temperatures, means for establishing and maintaining a gaseous atmosphere of specific content in one or the other or both of said muffle sections, and a short transition muifle section connecting said first and second sections and provided with means (1) for preventing transfer of heat from one section to the other and (2) for preventing migration of the gaseous atmosphere in from one of said first two sections to the other of first two sections.

Other objects and many of the attendant advantages of the present invention will become more readily apparent as reference is had to the following detailed description when considered together with the accompanying drawings wherein:

FIG. 1 is a fragmentary longitudinal section of a mufile embodying a preferred form of transition section constructed according to the present invention, the section being taken along line 11 of FIG. 2;

FIG. 2 is a cross-sectional view taken along line 22 of FIG. 1; and

FIG. 3 is a fragmentary cross-sectional view similar to FIG. 2 (but on an enlarged scale) of a second form of the invention.

Referring now to FIGS. 1 and 2, there is shown a portion of a mufile embodying the present invention. The muffle comprises two sections 2 and 4 which are joined by a novel transition section identified generally at 6. All three sections are made of a metal which is capable of withstanding relatively high temperatures. The two mufile sections 2 and 4 may be of different cross sections, or, as in the illustrated embodiment, they may be of identical cross section. In FIGS. 1 and 2, both mufile sections have a S-Sided construction, comprising parallel side walls 10 and 12, a horizontal bottom wall 14, and two inclined top walls 16 and 18. Disposed within each of these sections is a quartz or metal plate 20. Preferably, each plate is perforated as shown. The purpose of each plate 20 is to provide support to a moving conveyor belt 22 and also to act as a baflle which helps distribute gases along the length of the mufile section of which it forms a part. The

conveyor 22 may be made of a suitable, flexible, heat-resistant material, as, for example, wire mesh. However, it may also be made of a flexible, high-temperature metal alloy in strip form. Objects to be heat-treated are placed on the belt and are then transported through the muffle by the belt so as to be subjected to a controlled heat treat ment. Although not shown, each of the sections 2 and 4 is provided with means for maintaining therein a selected atmosphere during the time that objects are being treated within the furnace. The atmosphere may consist of one or more gases. The term gases as used herein covers true gases as well as vaporized liquids and metals, as, for example, oxygen, nitrogen, hydrogen, air, steam, vaporized arsenic, and vaporized antimony. The atmosphere may be at room pressure, or above or below it, depending upon the process involved. Moreover, the atmosphere may be relatively stagnant or circulating continuously. In this connection, reference is made to my copending application Serial No. 103,881 filed April 18, 1961, now Patent No. 3,086,764, entitled Tandem Furnace for specific details as to how gases are introduced to and removed from different stages of a mufile.

As seen in FIG. 1, the two muflle sections are surrounded by the turns of two different electric heating coils and 32. These coils are intended to be merely representative; the muffle may be heated by other types of electric heaters, and, in certain special cases, may also be heated by gas heaters. Irrespective of the type of heaters used, it is to be understood that they may be operated to raise the temperatures of muflle sections 2 and 4 to the same or different temperatures, depending upon the process desired to be executed.

The transition section 6 comprises two cylindrical plates 38 and 40, with former welded to muffle section 2 and the latter welded to muflle section 4. At its center, the two plates are provided with openings 42 which match the interior configurations of the muffle sections. Plates 38 and are spaced from each other by a continuous circular ring of pipe 52 which is interposed between them. Pipe 52 is welded as at 54 to the peripheral edges of two plates 38 and 40, with approximately half of the exterior surface of pipe being exposed to the space enclosed between the two plates 38 and 40. From the foregoing description, it will be appreciated that plates 38 and 40 cooperate with pipe 52 to define a chamber 56. Disposed within pipe 52 is a second continuous ring of pipe 58. Pipes 52 and 58 define chambers 60 and 62. The second pipe ring 58 is maintained in fixed relation to the outer pipe ring 52 by means of inturned portions 64 of the latter. These inturned portions 64 are welded to pipe ring 58.

Chamber 60 formed between communicates with chamber 56 by means of ports 68 formed in pipe 52. Also communicating with chamber 60 is a pipe 70. The latter is connected to a pipe 72. Also connected to the pipe 72 in parallel with the pipe is a pipe 74 which is attached to and communicates with the interior of pipe 58, i.e., chamber 62. Also communicating with chamber 62 is another pipe 76. It is to be observed that the pipes 74 and 76 pass through openings in pipe 52 and are Welded to the latter so as to form fluid tight seals. Pipe 76 functions to supply coolant under pressure to pipe 58 from a coolant source (not shown). The coolant may be any suitable liquid, but preferably water. The coolant flows around the interior of the pipe 58 and exits via the pipes 74 and 72. As the coolant exits from the pipe 72, it produces an aspirator effect on pipe 70, causing fiuid to be sucked out of chamber 60 and become entrained with the coolant. Thus, for example, if oxygen is present in chambers 56 and 60 and water is flowing through chamber 62, the departing water will suck oxygen out of chamber 62. The oxygen removed from chamber 62 will be replaced by more oxygen from chamber 56. The coolant leaving via pipe 72 may be discharged as waste or may be recirculated back into the pipe 76 after first allowing for escape of entrained gas.

Interposed between plates 38 and 40 and welded to pipe 52 is a circular metal plate 80 which is provided at its center with an opening 82 which is sized to accommodate the conveyor belt 22 and also to permit passage therethrough of articles which may be supported on the conveyor belt. Although only one metal plate 80 is shown, it is to be understood that more than one such plate may be welded to pipe 52. Where several parallel radiation plates are used in place of plate 80, it is preferred that they be spaced one from the other. As explained hereinafter, plate 80 functions as a radiation shield.

Operation of the muflle of FIGS. 1 and 2 will now be described with reference to an assumed heat treating process.

Assume for purposes of discussion that conveyor belt 22 is moving at a selected speed in the direction indicated by the arrow in FIG. 1 and that it carries one or more boats loaded with mating components for metal-to-glass seals wherein the metal component is Kovar. Assume further that a heat treatment to be executed in the illustrated muffle which comprises the steps of: (1) subjecting the boats and their contents while in mufile section 2 to an oxygen atmosphere and a temperature of approximately 750 C. for a predetermined time determined by the speed of the conveyor so as to give the Kovar a controlled coating of oxide, (2) subsequently heating the metal and glass components in mufiie section 4 in the presence of a nitrogen atmosphere at a temperature of 1100 C. so as to cause the glass to melt and fuse to the Kovar, and (3) thereafter cooling the completed seals outside of muffle section 4 under controlled conditions to ambient temperatures.

Under the foregoing conditions, there will be a tendency for the heat in muflle section 4 to transfer to the muffle section 2 so as to reduce the temperature differential between the two sections. There also will be a tendency for the gas in one muffie section to migrate to and mix with the gas of the other mufiie section, thereby diluting and contaminating the gas in the one muflle section. However, such transfer of heat and gas from one muffle section to the other is effectively prevented by the novel transition section 6. The latter transfer section 6 produces and maintains a sharp temperature gradient between muffle sections 2 and 4 and simultaneously acts to remove from the mufile any gas which tends to flow from one mufile section to the other muffie section.

The obtainment of a sharp temperature gradient is due entirely to the construction of transition section 6. First of all, little or no heat is transferred by conduction from muffle section 4 to muffle section 2 (or vice versa Where the temperature difference is reversed). The loop of coolant flow provided by the pipe 58 completely prevents transfer of heat by conduction, the circulating water substantially absorbing most of the heat which would be conducted from mufile section 4 to muffle section 2 by way of plates 40 and 38 and pipe 52. Secondly, little or no heat is transferred from muffle section 4 to muffle section 2 by radiation since heat which is radiated by plate 40 toward the opposite plate 38 will be intercepted by radiation shield and then transferred to pipe 58 by conduction, whereat it will be absorbed by the circulating water and removed from the system. Thirdly, the amount of heat which is or can be transferred by convection is negligible due to the fact that the gases have a relatively low specific heat and because no gas can migrate from one muflle section to the other. If the gases in either or both of the muffle sections 2 and 4 tend to move toward the transition section 6, they will be sucked out of the muffle via chamber 60 and pipes 70 and 72. The outflow of gas from pipe 70 will depend to a great extent on the velocity of the water flowing through pipes 74 and 72 and also to some extent on the relative sizes of the pipes 70 and 74. Suffice it to state that flow of water and the dimensions of the aforementioned component parts will be such that gas will be removed from transition section 6 as fast as it is received from the muffle sections 2 and 4. The resultant outflow of gas from the transition section 6 not only prevents any significant transfer of heat by convection but also prevents the gaseous atmosphere in one muflle section from being diluted or contaminated by the gaseous atmosphere in the adjacent muffle section. In this way, the limits of the oxygen and nitrogen atmospheres can be set precisely so that, by controlling the velocity of conveyor belt 22, it is possible to achieve controlled oxidation and fusion.

Of course, it is understood that other processes also may be executed with the apparatus of FIGS. 1 and 2. For example, the muffle may be used for chemical reduction, out-gassing, and gaseous diffusion. It is to be observed that the muffle may include more than one transition section 6, depending upon the number of different gaseous atmospheres to be used and the number of different temperature gradients which are required to be maintained in order to successfully execute the contemplated method.

FIG. 3 illustrates a second form of invention. In this case, the functions of the two concentric pipe rings are reversed. In FIG. 3, the liquid is circulated through the outer pipe ring 52A while the gas which is sucked out of the muffle sections passes through the inner pipe 58A. In this arrangement, the pipe 74A communicates directly with the outer pipe 52A while the pipe 70A communicates directly with the inner pipe 58A. Although not shown, it is to be understood that a water inlet pipe corresponding to pipe 76 of FIG. 2 also is provided, this pipe communicating directly with the outer pipe 52A. Any gas in chamber 56 (FIG. 1) will flow into pipe 58A via a plurality of ports 68A defined by bushings 90 which pass through the outer pipe 52A and are sealed to and anchored in suitable holes in pipe 58A. The radiation shield 80A is secured directly to the outer pipe 52A, being notched to accommodate the heads of the bushings 90. In this connection, it is to be appreciated that the diameters of the ports 68A in bushings 90 is greater than the thickness of the radiation shield 80A, so as to premit egress of gas from the chamber 56 into the pipe 58A.

The embodiment of FIG. 3 functions in the same manner as the one shown in FIGS. 1 and 2, preserving a sharp temperature gradient between the two mufile sections which it connects and simultaneously preventing migration of gas from one to the other of the mufile sections connected by transition section 6.

It is to be noted that the present invention provides isolation of the temperatures and atmospheres of two different successive muflle sections by means of structure which is relatively inexpensive and easy to fabricate and which does not necessitate intricate or peculiar cross-sectional configurations for the muffle which might complicate the construction or mode of operation of the conveyor which transports articles through the muffle or which might otherwise impede uniform distribution of gases in the individual stages of the muffle. On the other hand, the invention is not limited to muffles having the specific configurations illustrated in the drawings. The precise cross-sectional configurations of the muflie sections is irrelevant so far as the transition section is concerned. However, it is appreciated that other factors may necessitate a particular cross-sectional mufile configuration.

It is to be observed also that the transition section does not limit or affect the exterior construction of the furnace. The piping required to furnish coolant to transition section 6 is no more elaborate or difiicult to provide than is the piping for jackets for heating and cooling heretofore employed in muffle-type furnaces. A further important advantage is that the various elements of the muflie may be made of the same type of material, thereby avoiding problems of bonding and danger of rupture due to different coefficients of expansion. It is to be observed also that the construction of the transition section in no way limits the type of heaters which may be employed with the two muflle sections; nor does the transition section cause any complications in the nature or arrangement of the insulation which is generally required to be placed about the mufile so as to avoid transfer of heat from the muflie to the outside casing of the furnace. A further advantage of the transition section is that it does not occupy much space and that it obtains its intended objectives without any significant increase in the overall length of the mufile.

Other advantages of the present invention will be obvious to person skilled in the art. Similarly, it will be obvious to persons skilled in the art that the instant invention is susceptible to many changes and variations without departing from the principles outlined above. Thus, for example, the gas outlet pipes 70 and 70a need not be connected to the coolant outlet pipes in order to achieve withdrawal of gas from chamber 56. Instead, pipes and 70a may be connected to a separate unit, e.g., a suction pump, which is capable of exerting suction fully as well as the aspirator arrangement illustrated in the drawings. Such an alternative arrangement will not prevent the invention from functioning in the manner previously described.

Accordingly, this invention is not to be limited except by the following claims.

I claim:

1. A muffle comprising a transition section adapted to preserve a temperature gradient between two successive stages in said mufiie and also to prevent migration of gas from one to the other of said successive stages, said transition section comprising two plates in parallel spaced relation to each other, first hollow means secured to said plates at their peripheries and totally enclosing the space between said plates, second hollow means disposed within and partially filling said first hollow means, one of said hollow means having its interior communicating directly with said space, means for circulating a fluid through the other of said hollow means, and means cooperating with said circulating means for withdrawing gas from said one hollow means and said space.

2. A muflle as defined by claim 1 wherein said one hollow means is said first hollow means.

3. A mufile as defined by claim 1 wherein said one hollow means is said second hollow means.

4. A muflie as defined by claim 1 wherein said lastmentioned means comprises a conduit connecting said one hollow means and said circulating means which permits said gas to be sucked out of said one hollow means by the fluid circulating through said other hollow means.

5. A muflie comprising a first mufile section connected in series with a second mufiie section by means of a third relatively short transition muflle section having a larger cross-section than said first and second sections, said first and second muffle sections adapted to accommodate gaseous atmospheres, the aforesaid three mufiie sections defining a continuous unobstructed passageway to accommodate a continuous conveyor for transporting articles to be heat treated in the mufile, means for heating said first muffle section to a first temperature, means for heating said second mufiie section to a second temperature, said third section comprising two parallel plates provided with central openings with one of said plates secured adjacent its central opening to said first section and the other of said plates secured adjacent its central opening to said second section, a first pipe secured to said plates at their peripheries and totally enclosing the space between said two plates, a second pipe disposed within said first pipe, said second pipe having an inlet and an outlet whereby cooling fluid may be supplied to and removed from said second pipe for the purpose of removing heat from said transition section, and said first pipe having an inlet communicating with said space and an outlet communicating with the outlet of said second pipe, whereby gaseous atmosphere within said space may be sucked therefrom via said first pipe by the cooling fluid flowing out of the outlet of said second pipe.

6. A muflle comprising a transition section adapted to preserve a temperature gradient between two successive stages in said muffle and also to prevent migration of gas from one to the other of said successive stages, said transition section comprising two plates in parallel spaced relation to each other, a first pipe secured to and fully enclosing the space between said two plates, a second pipe disposed within said first pipe, one of said pipes having its interior communicating directly with said space, means for circulating a fluid through the other of said pipes, and means cooperating with said circulating means for withdrawing gas from said one pipe and said space.

and secured to one of said pipes, said third plate spaced from each of said two plates.

References Cited in the file of this patent UNITED STATES PATENTS Elsey Mar. 29, 1932 Schmidt et al Feb. 27, 1951 Beck June 26, 1962 

1. A MUFFLE COMPRISING A TRANSITION SECTION ADAPTED TO PRESERVE A TEMPERATURE GRADIENT BETWEEN TWO SUCCESSIVE STAGES IN SAID MUFFLE AND ALSO TO PREVENT MIGRATION OF GAS FROM ONE TO THE OTHER OF SAID SUCCESSIVE STAGES, SAID TRANSITION SECTION COMPRISING TWO PLATES IN PARALLEL SPACED RELATION TO EACH OTHER, FIRST HOLLOW MEANS SECURED TO SAID PLATES AT THEIR PERIPHERIES AND TOTALLY ENCLOSING THE SPACE BETWEEN SAID PLATES, SECOND HOLLOW MEANS DISPOSED WITHIN AND PARTIALLY FILLING SAID FIRST HOLLOW MEANS, ONE OF SAID HOLLOW MEANS HAVING ITS INTERIOR COMMUNICATING DIRECTLY WITH SAID SPACE, MEANS FOR CIRCULATING A FLUID THROUGH THE OTHER OF SAID HOLLOW MEANS, AND MEANS COOPERATING WITH 